var a = [1,2,3];
var b = [4,5,6];
function fn(arr1,arr2){
  let j = 0;
  for(let i = 0;i<arr1.length;i++){
    if (i%2==0) {
      a.splice(i+1,0,arr2[j])
      j++;
    }
  }
  return arr1;
}

var c = fn(a,b)

console.log(c);
